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UK scientists exploring the ocean floor in the Caribbean have discovered an "astounding" set of hydrothermal vents, the deepest anywhere in the world.

Deploying a remotely-operated vehicle (ROV) in the Cayman Trough, they stumbled across a previously-unknown site nearly 5000m below the surface.

Video pictures relayed live back to the research ship mounting the operation show spindly chimneys up to 10m high.

They are belching out dark water - "a stunning sight", one scientist said.

In the immense pressure of the sea three miles down, the ROV, known as ISIS, was gently steered around the vents, taking pictures and gathering samples.

One of the people "piloting" the ROV said seabed smokestacks remind him of "the industrial Midlands".

Hydrothermal vents are among the strangest features of the deep ocean and their existence was not known until the 1970s. Since then they have been discovered at about 200 sites around the world including the Southern Ocean and the Atlantic.

But it was only three years ago that vents were first detected in the Cayman Trough, a deep trench formed by the boundary between two tectonic plates. One set of vents, known as Beebe, was established as the deepest on record - until the discovery last night of another slightly deeper set nearby, at 4,968m. or about three miles.

Despite the hostile environment around the vents, they are home to a stunning array of species, such as this fireworm

The water being blasted from the newly-found vents was measured at 401C, making this set among the hottest on the planet.

The expedition, funded by the Natural Environment Research Council, is being run from a British research ship, the James Cook, named after the 18th century explorer who blazed a trail of discovery around the Pacific.

Now the ship bearing his name is using the latest technology to open up and understand an underwater world of eerie landscapes and unusual life forms.

A live stream of video is relayed back to a control room on board - where a cheer went up when the ROV's lights and high definition cameras picked out the new vents amid total darkness.

The team had been looking for a set first identified a year ago but their search took them unexpectedly into an entirely new field.

Vent shrimp have special organs that alert the animal to super-heated water

The tallest of vents reaches about 10m.

The chief scientist, Dr Jon Copley of the National Oceanography Centre, said the discovery of "astounding mineral spires" was a "complete surprise".

"We initially thought it was a site we'd been to before but it looked so different we thought it had changed. But eventually we realised it looked different because it was different," Dr Copley told BBC News.

"The beauty of working in the deep oceans is that you're always stumbling over things that are completely new.

"It's teaching us how little we know and for a few minutes it's not about the science, it's about the wonder of the planet, something that's been hidden for so long."

The ROV remained on station for nearly 24 hours - a typical length for a dive - before being returned to the James Cook bearing samples of water and wildlife.

For the biologists on board, the vents act as a highly unusual habitat with a massive contrast between the water from the vents measuring just over 400C, compared to the surrounding sea temperature of around 4C.

The narrow interface between the two extremes of water - sometimes as narrow as a few centimetres - provides a unique environment for an array of creatures.

Instruments on ISIS' arm were used to collect temperature data

Ghostly-white shrimp - clustered on the rocks in teeming crowds - appear to have lost the ability to see because their eyes are fused together.

Verity Nye is one of the researchers studying the blind shrimp, brought up in the ROV's containers, filled at the seabed.

"We don't think they have functioning eyes but they have a really unusual organ on their backs which is like a warning system for them to tell them when they're getting too hot so they don't get too close to the hot water from the vents.

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Expert Analysis: David Shukman on board the James Cook

"But we really don't know how life operates down there so we're still trying to understand it."

According to Dr Copley, the expedition has already yielded finds that are likely, after lab analysis back home, to prove to be new species including a white anemone and starfish.

Further dives are scheduled in the coming days, with Japanese and American researchers planning investigations here later in the year.

The scientists on the James Cook hope the research will eventually answer two key questions: why and how life evolved in such a seemingly hostile environment.

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